Light emitted from the sun has a wide variety of spectra from the ultraviolet region to the infrared ray region. Of these, infrared ray accounts for about 50% of solar light, and said infrared ray is mainly classified into near-infrared ray whose wavelength is close to that of visible ray (wavelength: about 750 to 2,500 nm), middle-infrared ray having a wavelength equal to or more than that wavelength (about 2,500 to 4,000 nm) and far-infrared ray (wavelength: about 4,000 nm or more). Such infrared rays (especially near-infrared ray) have longer wavelengths than that of ultraviolet light, and thus have small energy, whereas the infrared rays have significant thermal actions, and when the infrared rays are absorbed by a substance, they are emitted as heat to thereby cause temperature rising. Accordingly, infrared ray is also called heat ray, and by reflecting infrared ray (especially, near-infrared ray), for example, indoor temperature rising can be suppressed.
In recent years, due to increase of interest in countermeasures for energy saving, an attempt is made to decrease loads on cooling instillation by attaching a film that reflects the above-mentioned near-infrared rays onto windowpanes of buildings or vehicles to reflect the transmission of heat ray from solar light. On the other hand, if such near-infrared reflective film also reflects visible ray, which has a wavelength that is close to that of near-infrared ray, the transparency of the film cannot be ensured, and thus the film is colored. Accordingly, a near-infrared reflective film that selectively reflects near-infrared ray but transmits visible ray is considered to be preferable.
As such near-infrared reflective film that can selectively reflect near-infrared ray and transmit visible ray, for example, Patent Literature 1 discloses a near-infrared ray reflective substrate in which a near-infrared ray reflective film including low-refractive index dielectric films and high-refractive index dielectric films that are alternately layered is formed on a transparent sheet glass. The above-mentioned near-infrared ray reflective film is characterized in that 4 or more and 11 or less dielectric films are layered on at least one surface, and the visible ray transmittance defined in JIS R3106-1998 of the sheet glass on which the near-infrared ray reflective film is formed is 70% or more and has a maximum value of reflection of more than 50% in the wavelength region of wavelengths of from 900 nm to 1,400 nm. Furthermore, said near-infrared ray reflective film satisfies the following conditions: (1) when the dielectric films are sequentially counted from the surface of the sheet glass, and the maximum value among the refractive indices of the even-numbered layers is defined as nemax and the minimum value is defined as nemin, and the maximum value among the refractive indices of the odd-numbered layers is defined as nomax and the minimum value is defined as nomin, nemax<nomin or nomax<nemin, and (2) when the refractive index of the ith layer is defined as n1 and the thickness is defined as di, 225 nm≤ni·di≤350 nm with respect to infrared ray having a wavelength λ in the range of from 900 to 1,400 nm.
As is also described in paragraphs “0029” and “0030” of the above-mentioned Patent Literature 1, it is possible to reflect near-infrared ray by providing a structure in which low-refractive index dielectric films and high-refractive index dielectric films are alternately layered to thereby cause interference among the dielectric films (the above-mentioned condition (1)). Furthermore, in order to reflect near-infrared ray at a desired wavelength (900 to 1,400 nm), it is important to adjust the optical path difference (optical film thickness) ni·di to be ¼ of the above-mentioned desired wavelength (the above-mentioned condition (2)).
In the near-infrared reflective film (near-infrared reflective film) described in Patent Literature 1, a sharp reflection peak appears in the near-infrared region, but reflection peaks (especially peaks so-called ripples, which are repeating of high reflection and low reflection depending on wavelengths) sometimes appear also in the visible region. Therefore, reflection arises not only for near-infrared ray but also for visible ray, and thus, in some cases, the transparency of the infrared reflective film cannot be sufficiently ensured.
Therefore, methods for preventing the arising of reflection peaks that can arise in the visible region while retaining a reflection peak in the near-infrared region have been considered. For example, Patent Literature 2 discloses an infrared ray cut filter in which plural high-refractive index thin films formed of a high-refractive index material and plural low-refractive index thin films formed of a low-refractive index material are alternately layered is formed on a transparent substrate. At this time, the above-mentioned multilayer film is constituted by 16 or more and 32 or less of the above-mentioned thin film layers, and the first layer from the side of the above-mentioned transparent substrate of the above-mentioned multilayer film is the above-mentioned high-refractive index thin film, and the final layer of the above-mentioned multilayer film is the above-mentioned low-refractive index thin film. Furthermore, when a designed wavelength is defined λ, as the multilayer film is characterized in that the first and the second layers from the side of the above-mentioned transparent substrate of the above-mentioned multilayer film are formed by optical film thicknesses of (λ/4) or more, and the third to the predetermined layer from the side of the above-mentioned transparent substrate of the above-mentioned multilayer film are formed by optical film thicknesses of (λ/4) or less, the layers between the above-mentioned predetermined layer and the above-mentioned final layer are formed by optical film thicknesses of (λ/4) or more, and the above-mentioned final layer is formed by an optical film thickness of (λ/4) or less. Specifically, the multilayer film that constitutes the infrared ray cut filter is divided into four layer groups, and the values of the optical film thicknesses are shifted from λ/4 for the respective four layer groups. According to the infrared ray cut filter of Patent Literature 2, the literature describes that a transmittance property in which the transmittance gradually decreases at wavelengths from 550 nm to 750 nm can be obtained, and ripples that can arise in the visible region can be suppressed.